Method of sintering blast-furnace feed



Feb. 2, 1965 w. T. RILEY ETAL 3,168,393

METHOD oF SINTBRING BLAST-FURNACE FEED Filed sept. 7, 1961 f/arneyUnited States Patent Office 3,108,393 Patented Feb. 2, 1965 lVIETHOD FSINTERING BLAST-FURACE FEED William T. Riley, Thornton Township, CookCounty, and

Richard E. Stoll, Bloom Township, Cook County, Ill.,

assignors to United States Steel Corporation, a corporation of NewJersey Filed Sept. 7, 1961, Ser. No. 136,469 4 Claims. (Cl. 75-5) Thisinvention relates to an improved sintering method and apparatus.

Although the invention is not thus limited, our method and apparatus areparticularly useful in sintering iron oxide fines, such as ore or liucdust. Conventional practice in sintering these materials involvesfeeding a mix of moistened iron oxide fines, sinter returns andcarbonaceous fuel to a traveling grate, ignitingr the upper surface ofthe resulting bed shortly beyond the point of feeding, and drawingcombustion air downwardly through the bed. Combustion takes place in anarrow zone which moves downwardly from the point of ignition to aburnthrough point near the discharge end of the grate. Typically themoisture content of the mix as fed to the grate is about 8 to 12 percentby weight. In the portion of the bed near the feed end, moisturecondenses from the products of combustion as they pass downwardlythrough the relatively cool unsintered material beneath. Consequentlymoisture builds up in this portion of the bed and reaches a maximumcontent about 1 to 3 percent higher than its original content at aninversion point intermediate the length of the grate. We have observedthis additional moisture lowers the permeability of the bed to flow ofcombustion air and thus hinders the whole operation.

An object of the present invention is to provide a method and means foravoiding loss of permeability in a sinter bed caused by moisturecondensation and thereby increasing the capacity of a sintering machine.

A further object is to provide an improved sintering method andapparatus in which additional heat is supplied to a sinter bed at acritical location between the ignition point and the usual inversionpoint to minimize moisture condensation and thus prevent loss ofpermeability.

In the drawing:

FIGURE 1 is a somewhat diagrammatic side elevational view of a sinteringmachine which embodies additional heating means in accordance with ourinvention; and

FIGURE 2 is a graph which compares the relative permeability of a sinterbed `at different locations along its length.

FIGURE 1 shows a sintering machine, which is conventional apart from ouradditional heating means, and includes a supporting frame 10, an endlesstraveling grate 12 mounted on said frame and having a suitable drive, afeeder 13 above the grate near one end of the frame, an ignition hood 14just beyond the feeder, and a series of windboxes 15 beneath the upperflight of the grate. The feeder deposits a sinter mix on the grate whereit forms a bed B. In the example of an iron oxide sintering operation,the mix typically has a composition by weight on a a dry basis about asfollows:

Percent Iron ore fines 60 to 75 Sinter returns 20 to 40 Carbonaceousfuel to 7 Sufficient moisture is added to raise the level to the rangedesired, usually 8 to 12% depending on the ore. The machine also isequipped with a plenum chamber and a suitable fan (not shown) fordrawing a downdraft of combustion air through the bed and windboxes.

In accordance with our invention, we position a frame 16 across grate 12at a location intermediate the length thereof, as hereinafter explained.Frame 16 carries a series of burners 17 to which are connected suitablesources of fuel (preferably gaseous) and air. These burners supplyadditional heat to bed B to drive oif moisture which has condensed fromthe products of combustion as they pass through the relatively coolunsintered material beneath the combustion zone. In this manner weprevent the moisture content from building up to a value at which itseriously interferes with permeability of the bed.

As an example to demonstrate the benefits which result from practice ofour invention, we compared the bed permeability obtained on aconventional sintering machine with and without the additional heat. Weused a Dwight- Lloyd sintering machine which has a rated capacity of5000 tons per 24 hour day. The machine includes 2l windboxes on eightfoot centers, and thus has an effective grate length of 168 feet. Theplenum chamber is serviced by an induced draft fan designed to draw395,000 cubic feet per minute through the plenum chamber. We fed asinter mix of the following composition to the grate:

Percent Iron ore fines 47.0 Coke braize 5.5 limestone 8.0 Hot returns20.0 Cold returns 10.0

Moisture 9.5

The grate speed was 12 feet per minute, and the bed depth 12.5 inches.

Referring to FIGURE 2, the two curves show rates of air ow through thesinter bed at dierent locations along the grate starting with the eighthwindbox from the feed end. The solid line curve shows the rate of airflow Without additional heat, while the dotted line curve shows the ratewith additional heat in accordance with our invention. In each instanceWe measured the ow rates at one minute intervals with an anemometertraveling with the grate. As the solid line curve shows, the flow rateincreased steadily from the origin (eighth windbox) for a distance ofabout 24 feet, where it levelled, and it commenced to drop rapidly at adistance of about 40 feet from the origin. The ow rate reached a minimumor an inversion point at a distance about 58 feet from the origin, andthereafter increased rapidly. We positioned our burners over the bedjust ahead of the location where the rate of air flow commences to drop,that is, about 20 feet in advance of the inversion point. We appliedheat at the rate of about 150,000 B.t.u. per minute. In this manner wealtogether eliminated the dip in the air ow rate, as the dotted linecurve shows. We were able to increase the capacity of the sinteringmachine by about 11 percent.

In general we position our burners just ahead of the location at whichthe air flow rate tends to drop or about 15 to 20 feet in advance ofwhere the inversion point tends to form. We supply heat at a rate ofabout 22,000 B.t.u. per 1% moisture to be driven olf per ton of sintermix. We obtain the greatest permeability by driving off all the moisturein the unsintered material beneath the combustion zone, but wesuccessfully eliminate the inversion point by driving off enoughmoisture to lower the moisture content to its original value.

While we have shown and described only a single embodiment of theinvention, it is apparent that modifications may arise. Therefore, we donot wish to be limited to the disclosure set forth but only by the scopeof the appended claims.

We claim:

1. In a sintering process which includes feeding a moistcned sinter mixto the grate of a downdraft traveling grate sintering machine to form abed thereon, igniting the bed at its upper surface adjacent the point offeeding, and drawing Acombustion air downwardly through the bed, wherebycombustion takes place in a narrow zone which moves downwardly from thepoint of ignition to a burn-through point near the discharge end of thegrate, and whereby moisture in the products of combustion tends tocondense as these products pass through relatively cool unsintered mixbeneath said zone and the resulting additional moisture tends to createan inversion point intermediate the length of the bed, a method ofpreventing the formation of an inversion point comprising supplyingadditional heat to the bed to drive off condensed moisture from theunsintered material in advance of the location where the inversion pointtends to form.

2. A method as defined in claim 1 in which the heat is supplied by-burning gaseous fuel over the bed l5 Vto 20 feet in advance'of thelocation where the inversion point tends to form. Y v t 3. A method asdefined in claim 1 in which the heat is supplied by burning gaseous fuelover the bed l5 to 20 feet in advance of the location where theinversion point tends to form, and at a rate suicient to lower themoisture content at least to its original value as'a maximum.

4. In a sintering process in which a combustible sinter mix having amoisture content of 8 to 12 percent feeds to a single location on thegrate of. a downdraft traveling grate sintering machine and forms apermeable bed thereon, the bed is ignited at a single location on itsupper surface adjacent the location of feeding, and combustion air isdrawn downwardly through the bed, whereby combustion takes place in asingle narrow zone which moves downwardly from the location where thebed is ignited to a burn-through point near the discharge end of thegrate, and wherein moisture in the products of combustion condenses asthese products pass through relatively cool unsintered mix beneath saidzone to an extent that the permeability of the bed commences to drop ata location spaced from the location where the bed is ignited andultimately tends to reach an inversion point 15 to 20 feet beyond thelocation where permeability commences to drop, the combination therewithof a method of preventing the formation o f an inversion pointcomprising burning lgaseous fuel over the bed adjacent the locationwhere its permeability commences to drop and thus supplying additionalheat to the bed at a'rate sufficient to drive or condensed moisture fromthe unsintered Amix beneath said zone and lower the moisture contentthereof at least'to its original value as a maximum.

References Cited in the file of this patent UNITED STATES PATENTS1,292,059 Richards Jan. 2l, 1919 2,412,104 Stewart Dec. 3, 19462,498,766 Pettigrew Feb. 28, 1950 2,750,274 Leuep June 12, 1956

1. IN A SINTERING PROCESS WHICH INCLUDES FEEDING A MOISTENED SINTER MIXTO THE GRATE OF A DOWNDRAFT TRAVELING GRATE SINTERING MACHINE TO FORM ABED THEREON, IGNITING THE BED AT ITS UPPER SURFACE ADJACENT THE POINT OFFEEDING, AND DRAWING COMBUSTION AIR DOWNWARDLY THROUGH THE BED, WHEREBYCOMBUSTION TAKES PLACE IN A NARROW ZONE WHICH MOVES DOWNWARDLY FROM THEPOINT OF IGNITION TO A "BURN-THROUGH POINT" NEAR THE DISCHARGE END OFTHE GRATE, AND WHEREBY MOISTURE IN THE PRODUCTS OF COMBUSTION TENDS TOCONDENSE AS THESE PRODUCTS PASS THROUGH RELATIVELY COOL UNSINTERED MIXBENEATH SAID ZONE AND THE RESULTING ADDITIONAL MOISTURE TENDS TO CREATEAN "INVERSION POINT" INTERMEDIATE THE LENGTH OF THE BED, A METHOD OFPREVENTING THE FORMATION OF AN "INVERSION POINT" COMPRISING SUPPLYINGADDITIONAL HEAT TO THE BED TO DRIVE OFF CONDENSED MOISTURE FROM THEUNSINTERED MATERIAL IN ADVANCE OF THE LOCATION WHERE THE "INVERSIONPOINT" TENDS TO FORM.